zebra: Upon vrf deletion, actually release this data.

zebra: Upon vrf deletion, actually release this data.

When a vrf is deleted we need to tell the zebra_router that we have
finished using the tables we are keeping track of.  This will allow
us to properly cleanup the data structures associated with them.

This fixes this valgrind error found:

==8579== Invalid read of size 8
==8579==    at 0x430034: zvrf_id (zebra_vrf.h:167)
==8579==    by 0x432366: rib_process (zebra_rib.c:1580)
==8579==    by 0x432366: process_subq (zebra_rib.c:2092)
==8579==    by 0x432366: meta_queue_process (zebra_rib.c:2188)
==8579==    by 0x48C99FE: work_queue_run (workqueue.c:291)
==8579==    by 0x48C3788: thread_call (thread.c:1607)
==8579==    by 0x48A2E9E: frr_run (libfrr.c:1011)
==8579==    by 0x41316A: main (main.c:473)
==8579==  Address 0x5aeb750 is 0 bytes inside a block of size 4,424 free'd
==8579==    at 0x4839A0C: free (vg_replace_malloc.c:540)
==8579==    by 0x438914: zebra_vrf_delete (zebra_vrf.c:279)
==8579==    by 0x48C4225: vrf_delete (vrf.c:243)
==8579==    by 0x48C4225: vrf_delete (vrf.c:217)
==8579==    by 0x4151CE: netlink_vrf_change (if_netlink.c:364)
==8579==    by 0x416810: netlink_link_change (if_netlink.c:1189)
==8579==    by 0x41C1FC: netlink_parse_info (kernel_netlink.c:904)
==8579==    by 0x41C2D3: kernel_read (kernel_netlink.c:389)
==8579==    by 0x48C3788: thread_call (thread.c:1607)
==8579==    by 0x48A2E9E: frr_run (libfrr.c:1011)
==8579==    by 0x41316A: main (main.c:473)
==8579==  Block was alloc'd at
==8579==    at 0x483AB1A: calloc (vg_replace_malloc.c:762)
==8579==    by 0x48A6030: qcalloc (memory.c:110)
==8579==    by 0x4389EF: zebra_vrf_alloc (zebra_vrf.c:382)
==8579==    by 0x438A42: zebra_vrf_new (zebra_vrf.c:93)
==8579==    by 0x48C40AD: vrf_get (vrf.c:209)
==8579==    by 0x415144: netlink_vrf_change (if_netlink.c:319)
==8579==    by 0x415E90: netlink_interface (if_netlink.c:653)
==8579==    by 0x41C1FC: netlink_parse_info (kernel_netlink.c:904)
==8579==    by 0x4163E8: interface_lookup_netlink (if_netlink.c:760)
==8579==    by 0x42BB37: zebra_ns_enable (zebra_ns.c:130)
==8579==    by 0x42BC5E: zebra_ns_init (zebra_ns.c:208)
==8579==    by 0x4130F4: main (main.c:401)

This can be found by: `ip link del <VRF DEVICE NAME>` then `ip link add <NAME> type vrf table X` again and
then attempting to use the vrf.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>